1. Field of the Invention
The present invention relates generally to implantable devices. More particularly, the present invention is directed to a method and apparatus for coating an implantable device having a plurality of depots.
2. Description of the Related Art
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress the atherosclerotic plaque of the lesion against the inner wall of the artery to dilate the lumen. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the vessel after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, an implantable device, such as a stent, may be implanted.
Stents are scaffoldings, usually cylindrical or tubular in shape, which function to physically hold open and, if desired, to expand the wall of the vessel. Typically stents are capable of being compressed, so that they may be inserted through small cavities via catheters, and then expanded to a larger diameter once they are at the desired location.
Although stents are significant innovations in the treatment of occluded vessels, there remains a need for administering therapeutic substances to the treatment site. Systemic administration of the therapeutic substance often produces adverse or toxic side effects for the patient. Local delivery of therapeutic substances, by contrast, provides a smaller overall dosage that is concentrated at a specific treatment site. Local delivery can produce fewer side effects and achieve more effective results.
One technique for the local delivery of therapeutic substances employs medicated coatings on implantable devices. A typical method for medicating an implantable device includes applying a composition containing a polymer, a solvent, and a therapeutic substance to the implantable device using conventional techniques, for example, a dip-coating technique.
A recently developed type of stent includes a plurality of pores, called “depots” herein, that are formed in the outer surface of the stent. The depots are sized and shaped to contain the composition to ensure that a measured dosage of the composition is delivered with the stent to the specific treatment site. Unfortunately, when such stents are coated using conventional techniques, undesirable pockets of air can become trapped in the depots. The trapped air reduces the volume available in the depot for holding the composition. The uncertainty regarding the amount of composition loaded in the depots can cause inaccurate dosages being delivered to treatment sites and result in ineffective treatment.